feat: allow link options to be set to true and false

[GitRowin]

Closes #9396

Requires this and this to be updated if the SvelteKit maintainers are okay with merging this.

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[Rich-Harris]

Thank you! I'm in favour of this change, though we should probably also update these docs to recommend "false" instead of "off": https://kit.svelte.dev/docs/link-options#disabling-options

I also wonder if we should (non-breakingly) deprecate "off" in favour of "false", in other words print a warning when encountering it? This would make it easy to remove off in future, instead of allowing multiple ways of doing the same thing.

[Rich-Harris]

On second thoughts, we should release a version of SvelteKit that respects "false", then update HTMLAttributes accordingly, then deprecate off, otherwise people might get a warning telling them to make an update that will cause a red squiggly. So I think this PR is good to go as-is

[Rich-Harris]

opened sveltejs/svelte#8638

[GitRowin]

I am in favor of deprecating off sooner or later. It makes the link options less confusing ("Is there a difference between off and false?") and more consistent ("Why does off exist but not on?").

The off value is probably not used that frequently, so it shouldn't cause much annoyance.